Digital sampling is a technique used to visualize a time-varying waveform by capturing quasi-instantaneous snapshots of the waveform via, for example, a sampling gate. The gate is “opened” and “closed” by narrow pulses (strobes) in a pulse train that exhibit a well-defined repetitive behavior such that ultimately all parts of the waveform are sampled. The sampling implementation can either be real-time or equivalent-time, where real-time sampling refers to the case where the sampling rate is higher than twice the highest frequency content of the waveform under test (Nyquist sampling), while equivalent-time sampling uses an arbitrarily low sampling rate. However, equivalent-time sampling requires the measured waveform to be repetitive (in order to provide accurate signal reconstruction)—a fundamental limitation when compared to real-time sampling. The present invention is independent of the sampling rate, and hence, can be either real-time or equivalent-time sampling.
The recent advances in the field of optical communication with new, more complex, data modulation formats as a key technology has created a need for optical waveform characterization tools which are capable of extracting more information from the waveform than simply its power as a function of time.
In particular, many different modulation formats have been developed which use modulation of the phase of the optical carrier to encode the data to be transmitted. A few types of phase modulated signals have already been employed in commercial systems, such as differential phase shift keying (DPSK) and differential quaternary phase shift keying (DQPSK). For these differential modulation formats the data is encoded as the relative phase shift between consecutive symbols. In DPSK modulation schemes, for example, a π phase shift between bits represents a logical “1” and a zero phase shift represents a logical “0”. For DQPSK modulation, each symbol contains two bits of information by allowing four different relative phase changes between consecutive bits (e.g., 0, π/2, π and 3π/2).
FIG. 1 is used to further clarify the concept of phase-encoded modulation formats such as phase-shift keying (PSK), differential phase-shift keying (DPSK), and QPSK and DQPSK as defined above. For each type of modulation, the optical phase and amplitude of the data signal are visualized in constellation diagrams showing the optical field amplitude as the radial distance from origin R and the optical field phase as the angle φ. In FIG. 1, the logical marks (ones) and spaces (zeros) are represented as either absolute phase and amplitude levels (for PSK and QPSK formats, FIGS. 1(a) and (b), respectively), or as phase and amplitude transitions for the differentially-coded phase and amplitude levels (for DPSK and DQPSK formats, FIGS. 1(c) and (d), respectively). For D/QPSK each symbol contains, as shown, two bits of information. Therefore, four different logical phase and amplitude combinations are used to represent the “symbols” in either of these modulation format types.
It is to be noted that the amplitude of the data signal is constant for each of these phase-encoded modulation techniques. Hence, if only the power of the incoming signal is “detected” using a conventional photodetector-based o/e conversion device, the phase information will be lost. To extract the phase information, the signal needs to be mixed with an optical reference signal which converts the phase information into amplitude information. For differentially-modulated signals, delay interferometers (DIs), such as Mach-Zehnder interferometers (MZIs), Michelson interferometers, or the like, are commonly used in which the signal itself serves as reference after being delayed one (or more) bit periods. For absolute phase encoded signals (e.g. PSK or QPSK), an independent reference signal is necessary to extract the phase information from each bit.
The DI is an interferometric structure where the incoming optical waveform is split up (preferably equally) into two paths and one path is delayed relative to the second path before recombining the two paths. The relative delay is coarsely set equal to an integer number of bit slots (most commonly one bit slot) and finely tuned to match a particular relative phase delay of the optical carrier. For example, in the DPSK case, the relative delay is a multiple of π in order to effectively translate the relative phase shifts between the symbols into a binary amplitude modulated signal. The DI has two output ports—a constructive interference port and a deconstructive interference port (the ‘destructive’ port outputting the complementary data of the ‘constructive’ port). In order to optimize a DPSK receiver in terms of signal sensitivity, both outputs from the DI are detected by a so-called “balanced detector” structure.
In order to recover the data embedded in an incoming DQPSK signal, the signal is first evenly split so as to applied as “equal power” inputs into two separate DIs with different relative optical phase delays (+π/4+n*π and −π/4+m*π, where n and m are integers) and each DI pair of outputs is thereafter detected by a balanced detector structure. By properly choosing the relative phase delays, two bits of information per symbol can be separated and represented as one bit per balanced detector output. The amplitude modulated output from each balanced detector is thereafter sampled (for example, digital sampling) in order to visualize each bit's corresponding eye-diagram.
A major concern when using balanced detection for optical to electrical (o/e) conversion followed by electrical digital sampling is the influence of the measurement system on the measured waveform, which is known to introduce measurement error. In particular, balanced detection and electrical sampling suffer from two major limitations: (1) limited measurement bandwidth (currently <50 GHz); and (2) significant impedance mismatch, resulting in distortion in the measured waveform. For high speed signal characterization (10 GSymbols/s, 40 GSymbols/s or higher), these effects can influence the measurement results to such an extent that the measured waveform is dominated by the measurement system impulse response, which is unacceptable when needing to recover such high speed data signals.
Thus, a need remains in the art for an arrangement capable of characterizing (visualizing) high symbol rate optical signals without being hampered by the measurement system bandwidth or the distortion due to ole conversion and related impedance matching issues.